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Embryology  op 
THE   Eye 


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PREFACE.  (  ^93 


The  following  study,  carried  out  in  the 
New  York  Ophthalmic  and  Aural  Institute  at 
Prof.  Knapp's  suggestion,  is  based  upon  the 
examination  of  a  great  number  of  specimens. 
The  chick-embryos  were  obtained  by  incu- 
bation, and  were  hardened  in  Kleinenberg's  or 
Miiller's  fluid,  and  stained  with  carmine  and 
hematoxylin-eosin.  The  pig-embryos  were 
obtained  fresh  and  hardened  in  Midler's  fluid, 
after  decalcifying,  when  necessary,  with 
phloroglucin  or  hydrochloric  acid  mixtures, 
and  the  sections  were  stained  mostly  with 
hematoxylin-eosin.  Some  of  the  breaks  in  my 
series  were  supplied  by  Dr.  B.  Alex.  Randall 
of  Philadelphia  who  kindly  lent  me  a  number 
of  his  specimens,   prepared  by   Dr.    Piersol. 

W.  A.  H. 


37  West  39TH  Street,  New  York, 
Augtist   75-,    i8gj. 

iii 


AN    OUTLINE 

OF  THE 

EMBRYOLOGY   OF  THE    EYE 

WITH   ILLUSTRATIONS    FROM    ORIGINAL 
PEN-DRAWINGS   BY   THE   AUTHOR 


WARD    A.  HOLDEN,  A.M.,  M.D. 

lSSISTANT    surgeon    new    YORK    OPHTHALMIC    AND    AURAL    INSTITUTE 
CLINICAL    ASSISTANT    VANDERBILT    CLINIC 


THE  CARTWRIGHT  PRIZE  ESSAY  FOR  lS93 


G.  P.  PUTNAM'S   SONS 

NEW   YORK  LONDON 

27    WEST   TWENTY-THIRD    STREET  24    BEDFORD    STREET,    STRAND 

8;^£  ^nitkcrbochcr  ^rcss 
1893 


Copyright,  1893 

BY 

WARD  A.  HOLDEN 

Entered  at  Stationers'  Hall,  London, 
Bv  G.  P.  Putnam's  Sons 


lUectrotyped,  Printed,  and  Bound  bj- 

Cbc  Tknickerbocher  ipvcss,  IWew  HJorfe 

c;.  p.  Putnam's  Sons 


AN  OUTLINE  OF  THE  EMBRYOLOGY 
OF  THE  EYE. 


[The  drawings  referred  to  by  numbers  will  be  found  on  the  plates  at 
the  back  of  the  volume.] 


In  endeavoring  to  present  a  clear  and 
comprehensive  description  of  the  development 
of  the  eye,  it  has  seemed  to  me  best  to  give 
first  a  brief  and  purely  schematic  sketch  of  the 
processes  which  take  place,  explaining  them 
with  diagrams,  and  next  to  give  an  accurate 
histological  description  of  the  various  parts  of 
the  eye  in  their  successive  phases  of  develop- 
ment, illustrating  these  descriptions  with  care- 
ful drawings  from  actual  preparations.  It 
will  be  noticed  that  the  text  is  not  burdened 
with  frequent  references  to  the  voluminous 
literature  of  the  subject.  The  microscopic 
descriptions  are  the  unprejudiced  interpreta- 


2  THE  EMBRYOLOGY  OF   THE  EYE. 

tion  of  what  I  have  myself  seen.  When  some 
essential  point  has  not  been  shown  in  my 
specimens,  an  authority  that  has  described  it 
is  quoted.  In  regard  to  some  disputed  points 
the  views  held  by  different  authorities  are  given. 
The  earliest  periods  up  to  the  formation  of 
the  lens-sac  have  been  studied  in  the  embryo- 
chick,  and  the  later  periods,  on  account  of  the 
difficulty  in  obtaining  a  complete  series  of 
human  embryos,  have  been  studied  in  the 
foetal  rabbit  and  pig,  the  eye  of  the  latter 
closely  resembling  that  of  man. 

I. — A  Sketch  of  the  Processes  Occurring. 

At  an  early  stage  the  ovum  consists  exter- 
nally of  a  layer  of  closely  packed  cells  having  a 
definite  form  and  arrangement  (the  epiblast), 
and  beneath  or  internal  to  the  epiblast  of  a 
layer  of  loosely  connected  branched  cells  of 
irregular  form  (the  mesoblast).  As  the  hypo- 
blast does  not  take  part  in  the  development 
of  the  eye,  it  need  not  be  spoken  of  here. 

Very  early  in  the  development  of  the  epi- 


THE  EMBRYOLOGY  OF   THE  EYE.  3 

blast  a  linear  furrow  forms  upon  its  inner 
surface,  and  the  portion  of  epiblast  surround- 
ing this  furrow  becomes  thickened  so  as  to 
dip  down  into  the  mesoblast  and  push  the 
furrow  before  it.  At  length  this  furrow 
closes  and,  becoming  separated  from  the  ex- 
ternal epiblastic  layer,  forms  a  long  narrow 
tube,  the  neural  canal,  which  is  the  begin- 
ning   of    the    cerebro-spinal    axis    (Fig.    A). 


Fig.  a. 


This  tube  becomes  dilated  at  the  extremity 
where  the  brain  is  to  be  formed,  and  constric- 
tions divide  the  dilated  portion  into  three 
parts,  called  the  anterior,  middle,  and  poste- 
rior p7Hmary  cerebral  vesicles.  Later  the 
anterior  and  the  posterior  primary  vesicles 
each  divide  again,  forming  thus  five  secondary 
cerebral  vesicles. 


4  THE   EMBRYOLOGY  OF   THE  EYE. 

The  first  important  step  to  occur  in  the 
development  of  the  cerebral  system  is  the 
bulging  out  of  a  small  portion  of  the  lateral 
wall  of  the  anterior  primary  cerebral  vesicle 
on  either  side,  forming  a  cavity  called  the 
primary  optic  vesicle  (Fig.  B).     This  primary 


Fig.  B. 

optic  vesicle,  which  is  thus  formed  from  the 
involuted  or  neural  epiblast,  pushes  out  until 
it  reaches  the  external  epiblast.  At  the  spot 
where  the  vesicle  is  in  contact  with  the  exter- 
nal epiblast,  the  latter  becomes  thickened  and 
cupped  (Fig.  C).     This  cup  closes,  forming  a 


Fig.  C. 


THE  EMBRXOLOGY  OF    THE  EYE, 


5 


sac — the   lens-sac, — which    becomes   detached 
from  the   external    epiblast   (Fig.    D).      The 


Fig.   D. 


anterior  cells  of  this  sac  form  the  anterior 
layer  of  epithelium  of  the  lens,  while  the 
cells  of  the  posterior  layer  of  the  sac  de- 
velop into  the  nucleated  fibres  which  ex- 
tend forward  and  make  up  the  substance  of 
the  lens. 

As  the  external  epiblastic  layer  becomes 
cupped  to  form  the  lens,  the  distal  wall  of  the 
primary  vesicle  is  first  depressed,  and  then, 
inasmuch  as  the  vesicle  continues  to  grow,  it 
is  gradually  involuted  until  it  comes  to  lie  in 
contact  with  the  proximal  (posterior)  wall. 
The  double-walled  cup  thus  formed  is  called 
the  secondary  optic  vesicle  (Fig.  E). 


THE  EMBRYOLOGY  OF   THE  EYE. 


^^\H>V, 


Fig.  E. 

The  cupping  of  the  primary  optic  vesicle, 
however,  is  not  confined  to  its  distal  wall,  but 
also  takes  place  in  its  inferior  (ventral)  wall, 
so  that  the  secondary  optic  vesicle  has  a 
circular  opening  distally,  occupied  by  the  lens, 
and  a  cleft  inferiorly,  which  extends  back  into 
the  pedicle  connecting  the  optic  with  the  cere- 
bral vesicle.  This  pedicle,  or  optic  stalk,  is 
the  rudiment  of  the  optic  nerve  (Fig.  F). 


Fig.  F. 


Later  the  margins  of  this  cleft  unite,  and 
the  latter  is  obliterated.  In  the  mammalia, 
however,  before  the  cleft  closes,  mesoblastic 


THE  EMBRYOLOGY   OF   THE  EYE.  7 

tissue  passes  through  it  into  the  cavity  of  the 
secondary  vesicle.  Previously  a  small  quan- 
tity of  mesoblastic  tissue  has  been  carried  in 
with  the  lens  and  takes  part  in  the  formation 
of  its  vascular  sheath,  but  a  greater  quantity 
of  mesoblastic  tissue  passes  through  the  in- 
ferior cleft  and  forms  the  vitreous,  which  is 
an  almost  structureless  substance  containing 
blood-vessels.  The  vessels  entering  first  at 
the  posterior  extremity  of  the  cleft  divide 
into  two  systems  of  branches,  one  of  which 
runs  forward  in  the  vitreous  near  the  wall  of 
the  cavity,  and  another  runs  to  the  posterior 
pole  of  the  lens  where  it  breaks  up  into  in- 
numerable twigs  which  lie  in  the  vascular 
sheath  of  the  lens. 

The  cleft  in  the  optic  stalk  closes  about 
these  mesoblastic  vessels  and  they  come  to  lie 
in  the  centre  of  the  optic  papilla,  the  branches 
which  run  in  the  vitreous  being  obliterated 
before  birth. 

As  soon  as  the  secondary  optic  vesicle  is 
formed,  its  two  layers  begin  to  differentiate. 


8  THE  EMBRYOLOGY  OF    THE   EYE. 

the  internal  or  distal  layer  becoming  thicker 
to  form  the  retina,  and  the  external  or  proxi- 
mal layer  diminishing  to  a  single  stratum  of 
cells,  which  become  pigmented  and  form  the 
pigment-epithelium. 

While  this  differentiation  is  taking  place, 
the  mesoblastic  tissue  just  about  the  vesicle 
becomes  denser  and  many  blood-vessels  ap- 
pear in  it,  forming  the  rudiment  of  the  choroid. 

Mesoblastic  tissue  also  pushes  out  just  be- 
hind the  external  epiblastic  layer  and  forms 
the  rudimentary  cornea,  the  external  epiblastic 
layer  forming  the  epithelium.  A  single  layer 
of  unbranched  mesoblastic  cells  covering  the 
primitive  cornea  posteriorly  becomes  the 
corneal  endothelium,  and  this  secretes  Desce- 
met's  membrane. 

In  the  meantime  the  secondary  vesicle  grows 
much  faster  than  the  lens,  and  the  folded 
margins  of  the  vesicle-wall  which  remain  in 
contact  with  the  lens  are  drawn  inward,  thus 
beginning  the  formation  of  the  iris.  A  sheet 
from   the    surrounding    mesoblast   passes  out 


THE   EMBRYOLOGY  OF   THE   EYE.  9 

over  the  margin  of  the  vesicle-wall  and  becomes 
continuous  with  the  anterior  portion  of  the 
vascular  sheath  of  the  lens,  forming  the  pupil- 
lary membrane.  The  epiblastic  vesicle-wall 
and  mesoblastic  sheet  develop  simultaneously 
to  form  the  iris.  The  pupillary  membrane 
together  with  the  remainder  of  the  vascular 
sheath  of  the  lens  is  absorbed  before  birth. 

As  the  vesicle-wall  grows,  it  is  thrown  into 
a  fold  near  the  equator  of  the  lens,  and  this 
fold  becoming  filled  with  mesoblastic  tissue 
forms  the  ciliary  body.  Further  growth  of 
the  vesicle  in  an  equatorial  direction  produces 
a  number  of  meridional  folds,  which  form  the 
ciliary  processes. 

At  this  time  the  distal  wall  of  the  vesicle, 
which  in  its  posterior  portion  thickens  to  form 
the  retina,  thins  down  in  its  anterior  portion 
to  a  single  layer  of  cells.  This  layer  is  called 
the  pars  ciliaris  retinae  in  that  portion  which 
covers  the  ciliary  body,  and  becomes  pigmented 
and  forms  a  portion  of  the  uveal  layer  where 
it  covers  the  posterior  surface  of  the  iris. 


lO  THE   EMBRYOLOGY  OE   THE  EYE. 

The  lids  are  formed  by  folds  of  eplblast 
thrown  out  above  and  below,  into  which  meso- 
blastic  tissue  pushes.  These  folds  as  they 
develop  cover  the  cornea  and  finally  meet. 
The  epithelium  of  either  lid  margin  proliferates 
and  joins  with  that  of  its  fellow,  thus  connect- 
ing the  lids  securely  together  and  forming  a 
closed  sac,  the  conjunctival  sac.  This  sac  is 
lined  by  the  epiblastic  layer,  which  remains  as 
the  epithelium  of  the  conjunctiva.  The  con- 
nective-tissue portions  of  the  lid  are  derived 
from  the  mesoblast. 

The  lachrymal  duct  is  formed  from  the 
lachrymal  furrow  (Fig.  19,  A  and  B),  a  groove 
lined  with  epiblast,  extending  from  the  eye  to 
the  olfactory  opening.  This  groove  forms  a 
canal  and  becomes  separated  from  the  exter- 
nal epiblast. 

To  return  and  trace  the  development  of  the 
cerebral  vesicles.  Their  walls,  as  we  have 
seen,  consist  of  a  layer  of  involuted  epiblast  or 
neural  epiblast.  The  first  marked  change  to 
occur  is  that  the  posterior  portion  of  the  wall 


THE  EMBRYOLOGY  OF   THE  EYE.  II 

of  the  anterior  primary  vesicle — which  subse- 
quently becomes  the  second  secondary  cere- 
bral vesicle — bulges  out  to  form  the  primary 
optic  vesicle.  From  the  primary  optic  vesicle 
develop  the  pigment-epithelium  and  the  retina 
proper,  which  latter  may  be  divided  into,  ist, 
the  cerebral  layer  (nerve-fibres,  ganglion-cells, 
etc.),  conducting  elements,  and,' 2d,  the  layer 
of  modified  neural  or  sensory  epithelium  (outer 
nuclear  layer,  rods  and  cones),  which  makes 
up  the  percipient  elements  of  the  organ  of 
vision.  The  lens  and  conjunctiva  originate 
in  the  external  epiblast,  and  the  remaining 
structures  of  the  eye  arise  from  the  mesoblast. 
Soon  after  the  formation  of  the  optic  vesi- 
cle, the  auditory  vesicle  appears  near  the  fifth 
secondary  cerebral  vesicle.  The  auditory  vesi- 
cle, however,  is  not  formed  by  a  bulging  of 
the  neural  epiblast  composing  the  cerebral 
vesicle  wall,  but  by  an  indipping  of  the  ex- 
ternal epiblast,  forming  a  closed  sac  and  after- 
wards becoming  separated  from  the  external 
epiblast.      This    sac   becomes   the   epithelial 


12  THE  EMBRYOLOGY  OF   THE  EYE. 

lining  of  the  labyrinth — the  modified  sensory 
epithelium  forming  the  percipient  elements 
of  the  organ  of  hearing.  The  auditory  nerve 
pushes  out  to  it  from  the  brain  at  a  later  period. 
The  neural  epiblast  undergoes  a  thickening 
in  its  entire  extent  and  forms  the  brain  and 
cord,  the  cavity  remaining  as  the  cerebral  ven- 
tricles and  the  spinal  canal  The  first  vesicle 
forms  the  cerebral  hemispheres,  the  corpus 
callosum,  lateral  ventricles,  etc.;  the  second 
(thalamencephalon)  gives  rise  to  the  retina 
and  optic  nerves  by  means  of  its  prolongation, 
the  optic  vesicle,  while  its  lateral  walls  thicken 
to  form  the  optic  thalami,  its  roof  to  form  the 
pineal  gland,  its  floor  to  form  the  pituitary 
body,  and  the  cavity  remains  as  the  third  ven- 
tricle. The  early  embryonic  relations  under- 
go a  considerable  change,  however,  and  the 
optic  tracts  later  connect  the  eye  with  the  third 
cerebral  vesicle  (mesencephalon),  from  which 
arise  the  corpora  quadrigemina.  The  fourth 
vesicle  gives  rise  to  the  cerebellum  and  pons, 
and  the  fifth  to  the  medulla  oblongata. 


THE   EMBRYOLOGY  OF   THE   EYE.  1 3 

II. — Histological. 

I.  Primary  optic  vesicle. — The  wall  of  the 
primary  optic  vesicle  is  the  direct  continuation 
of  the  neural  epiblastic  layer  forming  the  wall 
of  the  cerebral  vesicles  (Fig.  i,  A).  For  a 
considerable  time  its  histological  structure 
does  not  change.  It  consists  of  cells,  some- 
what similar  to  ordinary  epithelial  cells,  in  a 
layer  from  three  to  five  cells  deep,  more  or 
less  regularly  superimposed  in  a  radial  direc- 
tion, the  limits  of  the  cell-bodies  being  mostly 
indistinct.  The  external  and  the  internal 
margins  of  the  layer  are  sharp  and  regular. 

Outside  the  cerebral  vesicles  lies  the  meso- 
blast  composed  of  branched  cells  not  closely 
packed,  and  containing  many  thin-walled  blood- 
vessels, which  lie  for  the  most  part  near  the 
wall  of  the  vesicles.  Covering  the  whole  is 
the  external  epiblast,  a  homogeneous  layer  of 
protoplasm  with  a  single  row  of  granular  nuclei. 

In  birds  the  primary  optic  vesicle  reaches 
the  external  epiblast,  there  being  no  meso- 
blastic  cells  between  the  two.     In  mammals, 


14  THE  EMBRYOLOGY  OF   THE  EYE. 

however,  it  has  been  questioned  whether  there 
is  not  a  continuous  thin  layer  of  mesoblastic 
tissue  separating  them. 

2.  Secondary  optic  vesicle. — The  first  step 
toward  the  development  of  the  secondary 
optic  vesicle  is  the  thickening  of  the  external 
epiblast  to  form  the  lens.  This  thickening 
takes  place  at  the  point  where  the  primary 
optic  vesicle  is  in  contact  with  the  external 
epiblast.  As  this  thickening  increases,  the 
adjacent  wall  of  the  primary  vesicle  is  pushed 
in.  This  is  shown  in  Fig.  2,  in  which  the 
section  is  excentric  so  that  the  beginning  lens- 
sac  is  cut  through  its  periphery  and  hence 
appears  simply  as  a  thickening  and  not  as  a 
cupping,  which  is  shown  in  Fig.  3.  In  Fig.  2 
the  section  also  passes  outside  the  pedicle  of 
the  vesicle,  and  the  latter  appears  therefore  as 
a  closed  sac. 

With  its  further  growth  the  outer  or  distal 
wall  of  the  primary  vesicle  becomes  still 
further  involuted  (Fig.  3),  although  for  a  time 
the  two  walls  are  not  in  contact  in  their  entire 


THE  EMBRYOLOGY  OF   THE  EYE.  25 

the  vessels  in  the  papilla  and  run  in  the  retina 
just  external  to  the  limitans  interna  (Fig.  lo  to 
the  left),  but  do  not  pass  through  the  limiting 
membrane  and  therefore  do  not  anastomose 
with  the  vessels  of  the  lens-sheath  or  of  the 
vitreous. 

The  blood  from  the  retinal  vessels  is  re- 
turned to  the  papilla.  The  blood  from  the 
vitreous-vessels  circulates  in  the  lens-sheath 
and  is  carried  out  of  the  eye  by  vessels  of  the 
iris,  which  anastomose  with  the  vessels  of  the 
anterior  portion  of  the  lens-sheath.  Most 
writers  have  considered  the  retinal  vessels  as 
being  at  first  one  system  of  the  vitreous-ves- 
sels. The  retinal  vessels  are,  however,  from 
their  first  appearance  always  separated  from 
the  vitreous  by  the  limitans  interna. 

The  vitreous  has  shown,  up  to  this  period, 
a  slightly  fibrillar  structure,  the  direction  of 
the  fibrillae  being  in  general  parallel  to  the 
margin  of  the  vitreous  (Fig.  9).  Now,  well- 
marked  coarser  striations  appear  about  the 
blood-vessels  and  run  parallel  to  them  (Fig.  1 2). 


26  THE  EMBRYOLOGY  OF   THE  EYE. 

The  vitreous  when  shrinking  in  the  process 
of  hardening  contracts  in  the  direction  of 
these  striations,  and,  becoming  detached,  takes 
with  it  the  layer  of  retinal  vessels.  When  this 
occurs,  the  anterior  portion  of  the  retina  Is 
also  often  detached,  ruptured  at  its  origin, 
and  drawn  inward.  The  folds  in  the  retina, 
which  are  almost  always  found  in  preparations, 
and  which  are  represented  In  Figs.  7,  8,  9,  10, 
are  probably  all  artificial,  though  many  writers 
have  described  them  as  normally  existing. 

Some  anatomists  have  described  the  vitreous 
as  consisting  of  concentric  layers,  others  have 
described  it  as  homogeneous.  The  striations 
along  the  vessels  certainly  divide  the  vitreous 
to  a  certain  extent,  but  I  cannot  confirm  the 
observations  of  those  who  have  described  the 
vitreous  in  the  embryo  as  showing  a  distinctly 
lamellar  structure. 

In  Fig.  G  we  have  a  diagram  showing  the  ves- 
sels in  a  half-grown  embryo,  A  being  the  hy- 
aloid artery,  B  the  system  of  vitreous  arteries, 
and  C  the  arteries  and  veins  of  the  retina 
which  are  detached  from  the  equator  forward. 


THE  EMBRYOLOGY  OF   THE  EYE, 


27 


Fig.  G. 

Along  the  vessel  A  and  the  system  of  vessels 
B  we  find  well-marked  striations,  so  that  the 
vitreous  in  such  a  section  is  apparently  divided 
into  four  lamella.  The  intervening  spaces 
show  no  striations.  If  the  section  be  cut 
somewhat  excentric,  we  have  the  vessels 
arranged    as    in    Fig.    H.       Here    the    stria- 


FiG.  H. 


28 


THE   EMBRYOLOGY  OF    THE  EYE. 


tions  are  seen  running  along  the  system  of 
vitreous  arteries  B,  and  crossing  them  at 
an  angle,  so  that  they  may  be  followed  a 
little  distance  from  the  vessels.  If  the  sec- 
tion be  cut  still  more  excentric  we  find  the 
appearance  shown  in   Fig.  I.     The  system  of 


Fig.  T. 


vitreous-arteries  here  lies  close  to  the  lens, 
which  is  cut  nearer  to  its  equator,  so  that  in  a 
thick  section  the  vessels  of  the  lens-sheath  may 
be  followed  some  little  distance.  Here  the 
striations  are  seen  near  the  system  of  vessels, 
B.  It  is  clear,  if  three  such  sections  were  ex- 
amined at  random  and  the  relative  location  of 
the  vessels  was   not  noted,  that  an  observer 


THE  EMBRYOLOGY  OF   THE  EYE.  29 

might  suppose  the  entire  vitreous  to  be  divided 
into  lamellae  by  the  striations,  and  this  mistake 
would  seem  to  have  been  often  made.  Further 
than  these  striations  along  the  vessels  I  be- 
lieve there  are  no  structural  elements  in  the 
vitreous  causinor  a  stratification  which  can  be 
demonstrated  with  the  microscope. 

d.  Zonula.  The  anatomy  of  the  zonula  is 
still  imperfectly  understood,  and  its  develop- 
ment is  still  more  uncertain.  Recent  writers 
(Czermak,  Berger,  Topolanski,  and  others) 
have  devoted  considerable  study  to  its  normal 
structure,  but  there  are  many  points  on  which 
there  is  not  general  agreement. 

At  the  time  when  the  ciliary  processes  are 
being  formed  the  retinal  vessels  stop  at  the 
place  where  the  retina  gradually  thins  down 
into  the  pars  ciliaris  (Fig.  13).  The  limitans 
interna,  however,  continues  over  the  ciliary 
body,  and  from  this  time  on  grows  gradually 
thicker,  forming  a  distinct  homogeneous  mem- 
brane on  the  pars  ciliaris.  The  superficial 
portion  of  the  vitreous  anteriorly  has  a  fibrillar 


30  THE  EMBRYOLOGY  OF   THE  EYE. 

Structure,  and  a  few  nuclei  or  round  cells  are 
seen  in  it,  and  also  on  and  often  apparently  in 
the  membrane  of  the  pars  ciliaris.  The  ciliary 
body  at  this  period  and  later  is  adherent  to  the 
lens-capsule,  and  when  the  two  are  forcibly 
torn  apart  the  membrane  sometimes  becomes 
detached  from  the  pars  ciliaris,  and  a  continu- 
ous membrane  with  nuclei  lying  on  it  may  then 
be  seen  extending  from  the  ciliary  body  to  the 
lens-capsule.  This  has  often  been  mistaken 
for  the  zonula. 

The  vessels  of  the  lens-sheath  begin  to 
atrophy  at  this  stage.  At  times,  when  the 
ciliary  body  is  torn  away  from  the  lens,  long 
spindle-cells  (which  may  be  atrophic  vessel- 
walls,  since  earlier  no  spindle-cells  are  found 
in  the  lens-sheath)  run  from  the  lens  to  the 
ciliary  body.  Later,  when  the  vitreous-vessels 
have  disappeared,  the  vitreous  is  here  very 
fibrillar,  and  is  adherent  to  the  limitans  interna 
just  anterior  to  the  ora  serrata.  Thick  bundles 
of  vitreous-fibrillae,  often  wavy,  run  in  a  curved 
line  from  the  ciliary  body  to  the  equator  of  the 


THE   EMBRYOLOGY   OF    THE   EYE.  3 1 

lens  and  continue  over  its  posterior  surface. 
No  distinct  anterior  hyaloid  or  limiting  mem- 
brane of  the  vitreous  can  be  found  here.  In 
the  space  anterior  to  and  outside  the  vitreous 
we  find  zonula-fibres  running  from  the  ciliary 
body  to  the  lens.  Farther  back  we  find  simi- 
lar fibres,  with  nuclei  lying  on  or  in  them, 
running  from  one  ciliary  process  to  another  or 
from  the  smooth  portion  of  the  ciliary  body  to 
a  ciliary  process,  having  their  origin  in  and 
beine  inserted  into  the  homogfeneous  mem- 
brane  of  the  pars  ciliaris. 

Whether  the  zonula-fibres  originate  from 
the  vitreous-fibrillae  entirely,  or  in  part  from 
cells  of  the  vascular  sheath  of  the  lens  is  not 
shown  by  my  preparations,  and  the  role  of  the 
nuclei  or  cells  which  are  found  in  the  vitreous 
at  a  late  stage  is  uncertain  in  my  mind.  Until 
very  recently  it  was  generally  believed  that 
the  zonula  was  derived  from  the  vitreous. 
Treacher  Collins,  however,  in  human  em- 
bryos, has  observed  spindle-cells  of  the  lens- 
sheath  stretch  out    to    form    nucleated   fibres 


32  THE  EMBRYOLOGY  OF   THE  EYE, 

which  later  lose  their  nuclei  and  become  zonula- 
fibres,  and  his  view  is  supported  by  the  study 
of  two  specimens  showing  congenital  anom- 
alies of  the  zonula.  I  cannot  see,  however, 
how  such  an  explanation  could  hold  good  for 
the  so-called  orbiculo-ciliary  and  the  inter-  and 
intra-ciliary  fibres  which  are  not  inserted  into 
the  lens-capsule. 

4.  Crystalline  lens, — In  Fig.  i  the  primary 
vesicle  A  is  in  apposition  with  the  external 
epiblast.  In  Fig.  2  the  external  epiblast  has 
become  thickened.  This  section  is  somewhat 
excentric  and  does  not  show  the  cupping,  which 
is  clearly  seen  in  Fig.  3.  According  to  the 
later  writers  a  similar  cupping  occurs  in  most 
mammals.  The  opening  shown  in  the  cup  in 
Fig.  3  soon  closes,  so  that  the  external  epi- 
blast again  forms  a  continuous  layer  and 
the  cup  becomes  a  closed  sac. 

This  junction  of  two  folds  with  the  union  of 
the  outer  and  inner  layers,  and  the  entire 
separation  of  the  two  layers  from  each  other, 
is  the  same   process   that   occurs  in  the  first 


THE  EMBRYOLOGY  OF   THE  EYE.  33 

involution  of  the  neural  epiblast  to  form  the 
cerebro-spinal  axis,  and  again  in  the  closure  of 
the  cleft  in  the  ventral  wall  of  the  secondary 
optic  vesicle. 

The  wall  of  the  lens-sac  consists  of  a  uni- 
form layer  of  epithelial  cells,  four  or  five  deep, 
both  in  the  chick  (Fig.  3)  and  in  the  rabbit 
(Fig.  4).  The  cells  of  the  posterior  layer 
lengthen  out  into  fibres.  The  beginning  of 
this  process  is  shown  in  Fig.  4  below.  These 
fibres,  developing  more  rapidly  in  the  centre, 
push  forward  until  they  reach  the  anterior 
layer,  and  thus  obliterate  the  cavity  of  the 
sac  (Fig.  6).  Each  fibre  has  a  single  nucleus, 
and  is  developed  from  a  single  cell.  In  the 
embryo-pig  at  the  time  when  the  lens  is  fully 
formed,  the  anterior  epithelium  is  four  or  five 
cells  deep  (Fig.  7).  The  short  fibres  which 
develop  at  the  angles  of  the  lens-sac  curve 
sharply  outward  to  reach  the  anterior  epi- 
thelium, those  nearer  the  posterior  pole  curve 
less,   and    those    arising   in   the   polar   region 

extend  forward  in  a  straight  line  (Figs.  6  and 
3 


34  THE  EMBRYOLOGY  OF   THE  EYE. 

7).  The  anterior  epithelium  at  this  stage  is 
continued  far  back  behind  the  equator  of  the 
lens,  and  the  nuclei  of  the  fibres  are  bunched 
together  in  the  form  of  a  crescent  with  its 
convexity  forward.  As  the  lens  develops,  the 
cells  farthest  back  in  the  anterior  epithelial 
layer  become  lens-fibres  in  their  turn,  and  the 
limit  of  the  epithelial  layer  gradually  moves 
forward  (Figs.  7,  8,  9).  At  the  same  time  the 
layer  decreases  in  thickness  until  it  comes  to 
consist  of  a  single  stratum  of  cells,  which  ter- 
minate near  the  equator  of  the  lens  (Fig.  9), 
and  which  are  permanent.  The  nuclei  of  the 
fibres  become  more  scattered,  and  lose  to 
some  extent  their  regular  arrangement  (Fig. 
8),  and  the  curve  of  the  individual  fibres 
changes,  those  near  the  equator  of  the  lens 
curving  but  slightly  outward,  and  those  nearer 
the  axis  curving  slightly  inward.  This  change 
in  curvature  increases,  and  in  the  matured 
lens  most  of  the  fibres  curve  sharply  inward 
and  run  concentrically. 

From    the  time    when  the  lens-sac  is  first 


THE  EMBRYOLOGY  OF   THE  EYE.  35 

formed  it  is  surrounded  by  a  capsular  mem- 
brane which  gradually  increases  in  thickness. 
It  is  to  be  considered  a  cuticular  formation  of 
the  epithelial  cells,  and  it  becomes  much 
thicker  anteriorly  where  the  epithelial  cells 
remain  permanently.  Anteriorly  also  it  may 
be  split  up  into  lamellae,  each  of  which  repre- 
sents in  all  probability  a  certain  period  of 
formation.  The  vascular  sheath  is  for  the 
nutrition  of  the  lens  during  its  rapid  growth, 
and  seems  to  play  no  role  in  the  formation  of 
the  capsule  (cf.  p.  20). 

5.  Cornea. — When  the  lens-sac  becomes 
separated  from  the  external  epiblast,  an  open 
space  remains  between  the  two  (Fig.  6).  In 
the  rabbit  (Fig.  4)  the  mesoblastic  tissue 
passes  over  the  margin  of  the  secondary  optic 
vesicle  into  the  cavity  of  the  latter.  This 
mesoblastic  tissue  contains  many  vessels,  and 
at  this  stage  both  the  anterior  space  and  the 
cavity  of  the  vesicle  are  filled  with  a  fluid 
which  is  probably  transuded  from  the  vessels. 

Mesoblastic  cells  now  push  out    from  the 


36  THE  EMBRYOLOGY  OF   THE  EYE. 

sides  and  cross  the  space  between  the  external 
epiblast  and  the  lens-sac.  A  layer  of  meso- 
blastic  cells  is  thus  formed,  which  may  rest  on 
the  posterior  surface  of  the  external  epiblastic 
layer,  or  which  may  at  first  lie  slightly  pos- 
terior to  the  external  epiblast.  After  the  first 
continuous  layer  has  been  formed  from  the 
mesoblast  at  the  sides,  it  gradually  increases 
in  thickness,  the  new  cells  having  their  origin 
probably  in  the  division  both  of  the  cells  of 
the  existing  layer  and  of  the  cells  at  the  sides. 
These  cells  become  long  spindle-cells,  and  the 
anterior  cells,  if  at  first  they  have  not  rested 
on  the  external  epiblast,  soon  do  so,  and  there 
is  then  no  trace  of  any  intervening  material 
between  corneal  stroma  and  epithelium.  For 
a  time  the  rudimentary  cornea  is  much  thicker 
at  its  periphery  (Fig.  7).  At  length  the  cen- 
tral portion  fills  up  until  the  cornea  is  of  a 
uniform  thickness.  Then  a  single  stratum  of 
endothelial  cells,  the  origin  of  which  is  uncer- 
tain, passes  out  over  its  posterior  surface 
(Fig.  8). 


THE  EMBRYOLOGY  OF   THE  EYE.  37 

The  endothelial  layer  lies  on  the  stroma  of 
the  cornea  for  a  time.  Later  in  foetal  life  a 
delicate  double-contoured  membrane  appears 
between  them.  This  membrane  gradually 
thickens  until,  in  the  grown  animal,  it  is  a 
thick  lamina  which  may  be  stripped  off  cleanly, 
and  which  is  very  resistant  to  pathological 
processes  and  does  not  stain  like  the  stroma 
of  the  cornea.  It  is  to  be  considered  a 
product  of  the  endothelium.  When  the  en- 
dothelial layer  in  the  human  eye  is  displaced, 
it  may  proceed  to  the  development  of  a  new 
elastic  membrane,  altogether  similar  to  the  nor- 
mal membrane  of  Descemet  (Wagenmann). 

In  mammals,  as  we  have  seen,  the  stroma 
cells  of  the  rudimentary  cornea  lie  directly  on 
the  posterior  surface  of  the  epithelium  (Fig. 
7).  The  cells  become  spindle-shaped  and 
fibres  are  formed,  between  the  bundles  of 
which  the  cells  remain  permanently  as  the 
fixed  corneal  corpuscles.  When  the  cells 
have  become  long  spindle-cells  we  find  that 
no  nuclei  lie  on  the  exterjial  epithelium,  but 


38  THE  EMBRYOLOGY  OF   THE  EYE. 

that  the  nearest  nuclei  are  separated  from  the 
epithelium  by  a  fibrous  layer  similar  to  the 
corneal  stroma  as  the  stroma  appears  some- 
what later.  This  layer  after  It  Is  formed  has 
about  the  thickness  of  a  corneal  lamella,  and 
does  not  Increase  In  thickness  In  later  foetal 
life  like  the  membranes  of  cuticular  formation. 
It  has  a  sharp  outline  next  the  epithelium,  and 
the  epithelium  is  readily  stripped  cleanly  off 
it,  but  It  is  not  at  any  period  sharply  limited 
from  the  corneal  stroma  and  It  cannot  be 
readily  separated  from  the  latter.  It  stains 
much  the  same  as  the  corneal  stroma,  and  is 
similarly  affected  by  pathological  processes. 
It  would  seem  to  be  an  early  product  of  the 
corneal  cells,  being  fully  formed  at  a  time 
when  the  rudimentary  cornea  Is  still  com- 
posed of  splndle-cells,  and  distinct  fibrous 
tissue  has  not  yet  made  Its  appearance  else- 
where. 

In  birds  the  development  of  Bowman's 
membrane  Is  different.  The  space  between 
the  lens  and  the  external  epithelium  Is  filled 


THE  EMBRYOLOGY  OF   THE  EYE.  39 

in  the  chick  with  a  homoo^eneous  non-cellular 
material,  and  the  mesoblastic  cells  which  push 
out  to  form  the  stroma  of  the  cornea  do  not 
rest  directly  on  the  posterior  surface  of  the 
external  epithelium,  but  lie  a  little  distance 
from  it.  A  layer  of  the  homogeneous  material 
remains  between  them,  and  this  soon  appears 
as  a  double-contoured  membrane  which  is 
permanent.  The  epithelium  often  cannot  be 
stripped  off  it  cleanly,  and  fragments  of  the 
cells  remain  adherent  to  the  membrane.  This 
homogeneous  material  is  probably  a  transu- 
dation from  the  vessels.  The  mesoblastic 
cells  push  out  into  it,  and  for  a  time  it  forms 
the  intercellular  matrix  of  the  cornea. 

The  process  occurring  in  birds  has  been 
supposed  by  many  to  hold  good  for  mammals. 
In  my  opinion  the  development  of  Bowman's 
membrane  is  entirely  different  in  the  two. 
The  first  stroma-cells  of  the  cornea  in  mam- 
mals have  been  often  described  and  pictured 
as  lying  some  distance  behind  the  epithelium. 
This  may  sometimes  be  found  at  the  beginning 


40  THE  EMBRYOLOGY  OF   THE  EYE. 

of  the  corneal  development.  But  after  the 
rudimentary  cornea  is  a  few  cells  deep,  the 
anterior  cells  lie  directly  on  the  epithelium. 
In  birds  the  stroma-cells  do  not  reach  the 
epithelium  at  any  period. 

There  has  been  much  discussion  as  to 
whether  the  corneal  cells  are  cells  which  have 
wandered  in  from  the  mesoblast,  or  whether 
they  have  been  produced  in  loco.  Some 
authors  have  described  the  cornea  as  consist- 
ing in  the  beginning  of  a  non-cellular  homo- 
geneous material  into  which  the  corneal  cells 
pass  by  migration.  Others  have  described  it 
as  a  homogeneous  material  containing  a  few 
cells  which  by  their  division  produce  the  cor- 
neal cells.  In  mammals,  the  homogeneous 
material  need  not  be  taken  particularly  into 
account.  It  is  in  all  probability  simply  an  in- 
different fluid  filling  a  cavity.  The  mesoblas- 
tic  cells  at  the  sides  proliferate  and  thus  a  thin 
layer  of  cells  is  pushed  through  the  cavity. 
The  cells  of  this  layer  and  the  cells  at  the 
sides   continue    to    proliferate    and   form   the 


THE  EMBRYOLOGY  OF   THE   EYE.  4 1 

rudimentary  cornea.  When  the  mesoblastic 
cells  first  push  In  from  the  sides  to  form  the 
rudimentary  cornea  (Figs.  4  and  7),  they  have 
an  oval  nucleus  and  a  large  cell-body  with  two 
processes  and  occasionally  more.  In  a  vertical 
section  of  the  cornea  the  cells  have  a  parallel 
direction,  and  appear  as  separate  bipolar  cells, 
In  a  flat  section  the  processes  extend  in  various 
directions,  and  small  offshoots  from  the  pro- 
cesses anastomose  with  those  of  other  cells. 
Very  soon  the  cells  become  flattened.  Then 
in  a  vertical  section  of  the  cornea  the  nucleus 
has  a  long  spindle  form,  and  the  body  of  the 
cell  appears  only  as  a  long  delicate  process 
from  either  end  of  the  nucleus.  In  a  flat 
section  at  this  stage  we  see  a  slightly  oval 
nucleus  and  a  broad  cell-body  ending  in  tw^o 
sharp  processes.  We  now  find  delicate  fibres 
resting  on  the  cells,  appearing  similar  to  the 
delicate  cell-processes  as  seen  in  a  vertical 
section.  These  fibres  rapidly  increase  in  num- 
ber, separating  the  cells  in  a  vertical  direction. 
The  long  processes  of  the  cells  finally  become 


42  THE  EMBRYOLOGY  OF   THE  EYE. 

shorter,  and  the  cells  remain  as  fixed  corneal 
corpuscles  between  the  lamellse  of  fibres. 

The  external  epithelium  for  a  considerable 
time  consists  of  a  single  stratum  of  cuboidal 
cells  (Figs.  6  and  7).  Then  a  stratum  of  flat- 
tened cells  appears  external  to  the  cuboidal 
layer  (Fig.  8).  From  this  time  on,  the  epi- 
thelium gradually  thickens,  the  cells  of  the 
external  layers  remaining  flat,  those  of  the 
middle  layers  irregularly  cuboidal,  while  those 
of  the  inner  layers  are  columnar.  Just  beneath 
Bowman's  membrane  vessels  are  seen  near  the 
periphery  of  the  cornea,  continuous  with  con- 
junctival vessels. 

6.  Sclera. — At  the  time  when  the  lens  is  beinor 

o 

formed,  the  mesoblastic  tissue  outside  the  sec- 
ondary optic  vesicle  consists  of  a  mass  of  con- 
nective-tissue cells  of  various  forms,  the  nucleus 
being  round  or  nearly  so,  and  the  cells  having 
no  particular  arrangement.  Scattered  through 
this  tissue  are  thin-walled  blood-vessels,  and  a 
network  of  these  vessels  lies  on  the  vesicle- 
wall  (Figs.  3,  6).     A  little   later  these  vessels 


THE  EMBRYOLOGY  OF    THE   EYE.  43 

come  to  form  a  simple  layer  of  capillaries  rest- 
ing on  the  vesicle-wall  (Fig.  8).  The  meso- 
blastic  cells  then  become  spindle-shaped  and 
run  parallel  to  the  vesicle-wall,  and  later  the 
spindle-cells  form  fibres.  In  Fig.  lo  we  see 
outside  the  vesicle-wall  this  thin  fibrous  layer, 
which  is  the  rudimentary  sclera.  External  to 
it  and  at  this  period  not  sharply  differentiated 
from  it  is  the  general  mass  of  indifferent  meso- 
blastic  cells  filling  the  orbital  cavity.  A  little 
later  branched  pigmented  cells  appear  at  the 
outer  margin  of  the  rudimentary  sclera,  sep- 
arating it  from  the  orbital  tissue.  These  pig- 
mented cells  are  rarely  found  in  the  human 
sclera. 

External  to  the  pigmented  cells  is  a  thin 
layer  of  loosely  meshed  cells,  and  external  to 
these,  particularly  in  the  anterior  segment,  we 
find  a  very  thin  fibrous  layer  similar  to  the 
sclera,  which  represents  the  capsule  of  Tenon. 
Anteriorly  the  fibres  of  the  sclera  are  continu- 
ous with  those  of  the  cornea  (Fig.  9).  The 
loose  tissue  external   to  the  sclera  continues 


44  THE  EMBRYOLOGY  OF   THE  EYE. 

forward  as  the  subconjunctival  tissue,  a  loose 
meshwork  of  cells  with  many  blood-vessels. 

7.  CJioi^oid. — The  fibres  of  the  rudimentary 
sclera,  as  we  have  seen,  lie  at  first  just  exter- 
nal to  the  capillary-vessel  layer,  which  latter 
rests  upon  the  vesicle-wall  (Fig.  10).  Internal 
to  this  fibrous  layer  the  choroid  is  formed 
about  the  capillary  vessels. 

At  this  time  we  find  the  outer  layer  of  the 
vesicle-wall  consisting  of  a  single  stratum  of 
hexagonal  cells,  darkly  pigmented  with  rod^ 
shaped  particles  of  pigment.  This  stratum  is 
the  pigment-epithelium.  The  inner  margin  of 
this  stratum  is  irreo^ular  ;  its  outer  maro^in  is 
smooth,  and  resting  on  this  outer  margin  may 
be  seen  the  beginning  of  a  delicate  membrane, 
which  keeps  growing  thicker  throughout  the 
entire  course  of  foetal  life.  In  the  grown  ani- 
mal this  is  a  fairly  thick  homogeneous  mem- 
brane, the  so-called  lamina  vitrea  of  the 
choroid.  This  membrane  would  seem  to  be 
secreted  by  the  pigment-epithelium. 

Directly  on  this  thin  homogeneous  membrane 


THE  EMBRYOLOGY  OF   THE   EYE.  45 

rest  the  capillaries.  The  first  change  that  oc- 
curs in  the  formation  of  the  choroid  is  the 
arraneement  of  a  considerable  number  of  cells 
of  various  shapes  about  the  capillaries  and 
the  development  of  a  number  of  larger  vessels 
just  external  to  the  capillaries.  These  changes 
all  take  place  between  the  pigment-epithelium 
and  the  fibrous  layer  which  is  the  rudiment- 
ary sclera,  the  latter  being  simply  pushed 
outward  by  the  development  of  the  choroid 
beneath  it. 

After  the  appearance  of  the  larger  vessels, 
pigmented  cells  are  seen  just  beneath  the  ru- 
dimentary sclera  at  the  outer  margin  of  the 
rudimentary  choroid,  and  later  pigmented 
cells  are  found  deeper  in  the  choroid,  until 
finally  in  the  pig  they  reach  up  to  the  capil- 
lary layer.  In  man  the  pigmented  cells  seldom 
extend  into  Sattler's  layer  of  smaller  vessels, 
and  almost  never  reach  the  capillaries.  The 
pigment  is  in  the  form  of  rods,  shorter  and 
more  rounded  at  the  ends  than  the  rods  in  the 
pigment-epithelium. 


46  THE  EMBRYOLOGY  OF   THE   EYE. 

With  the  appearance  of  the  pigmented  cells 
the  vessels  become  more  numerous  and  the 
pigmented  and  unpigmented  cells  form  a 
delicate  meshwork  about  them.  The  external 
cells  form  long  fibres,  which  are  readily  de- 
tached from  the  rudimentary  sclera,  and 
among  these  fibres  free  endothelial  cells  are 
found. 

8.  Iins,  ciliajy  body,  and  pupillary  membrane. 

a.  The  iris  and  ciliary  body  are  composed 
of  an  epiblastic  and  a  mesoblastic  portion, 
each  of  which  may  be  considered  separately. 

Epiblastic  portion.  In  Fig.  3  we  see  the 
folded  anterior  margin  of  the  secondary  optic 
vesicle,  to  the  left  the  two  layers  some  dis- 
tance apart,  to  the  right  the  two  layers  in  ap- 
position. In  Fig.  7  the  outer  layer  has  taken 
on  its  pigment,  and  the  Inner  layer  has  become 
thinned  near  the  margin.  The  folded  margins 
turn  it  toward  the  lens,  and  in  Fig.  8  they  are 
in  contact  with  it.  At  this  stage  we  may 
notice  a  slight  curving  of  the  vesicle-wall  near 
its  folded   margin,  which  becomes  later  a  dis- 


THE  EMBRYOLOGY  OF    THE   EYE,  47 

tinct  fold   where   the  ciHary   body    is    to    be 
formed. 

The  inner  layer  of  the  vesicle-wall  becomes 
thinner  in  its  anterior  portion  (Figs.  8  and  9), 
until,  like  the  outer  layer,  it  comes  to  consist 
of  a  single  stratum  of  cells — the  pars  ciliaris 
retinae  (Fig.  11).  Farther  back  the  inner 
layer  is  thicker  and  passes  imperceptibly  over 
into  the  rudimentary  retina.  Anteriorly  the 
single  stratum  of  cells  becomes  pigmented  in 
the  portion  that  is  to  form  the  posterior  layer 
of  the  iris,  the  pigmentation  ceasing  at  the 
head  of  the  ciliary  body  (Fig.  11).  In  speci- 
mens from  the  grown  pig,  bleached  by  Collins' ' 
method,  the  posterior  layer  of  the  iris  with  the 
pigment  removed  consists  of  cells  entirely 
similar  to  those  of  the  pars  ciliaris,  the  only 
difference  being  that  the  layer  on  the  iris  does 
not  have  a  smooth  posterior  surface,  but  forms 
a  succession  of  short  curves. 

Some  time  after  the    development    of    the 
equatorial  fold  to  form  the  ciliary  body,  meridi- 

1  Trans.  Ophthal.  Soc.   Unit.  Kingdom,  vol.  xi. 


48  THE  EMBRYOLOGY  OF    THE  EYE. 

onal  folds  are  thrown  out  to  form  the  cIHary 
processes. 

The  mesoblastic  portion  plays  in  the  begin- 
ning a  more  passive  role  in  the  development 
of  the  iris  and  choroid.  In  Fig.  7  the  rudi- 
mentary cornea  and  the  general  mass  of  meso- 
blastic  tissue  are  seen  to  be  entirely  cellular. 
In  Fig.  8  the  mesoblastic  tissue  next  the 
vesicle-wall  is  still  cellular,  more  externally  it 
is  beginning  to  be  fibrous.  In  Fig.  9  the 
fibrous  layer  is  only  separated  from  the 
vesicle-wall  by  the  capillaries  of  the  rudi- 
mentary choroid.  The  mesoblastic  tissue  of 
the  rudimentary  iris  at  this  stage  forms  a  thick 
cellular  layer  with  many  blood-vessels,  some 
of  which  are  continuous  posteriorly  with  the 
choroidal  vessels  and  anteriorly  with  the  ves- 
sels of  the  lens-sheath.  This  mesoblastic 
cellular  tissue  later  fills  up  the  folds  thrown 
out  in  the  epiblastic  portion,  and  forms  the 
connective-tissue  portion  of  the  iris  and  ciliary 
body. 

In  Figs.    II   and  13  we  see  a  wedge-shaped 


THE   EMBRYOLOGY  OF    THE   EYE.  49 

mass  of  cells  running  transversely,  which  is 
intercalated  between  the  sclera  and  the  ciliary- 
body,  and  encloses  the  angle  of  the  anterior 
chamber.  These  cells  form  a  meshwork  of 
fibres  which  becomes  the  ligamentum  pecti- 
natum. 

The  layer  of  closely  packed  endothelial  cells 
on  the  posterior  surface  of  the  cornea  stops, 
as  a  layer,  at  the  angle  of  the  anterior  cham- 
ber, but  isolated  endothelial  cells  lie  in  the 
spaces  of  the  ligamentum  pectinatum,  and  a 
thin  endothelial  layer  with  scattered  nuclei 
continues  over  the  iris  and  out  into  the 
pupillary  membrane. 

The  sphincter  iridis  and  the  ciliary  muscle 
develop  late.  The  later  stages  of  develop- 
ment of  the  iris  and  ciliary  body  differ  con- 
siderably in  the  pig  from  those  in  man,  and 
are  of  no  particular  interest  here. 

b.  The  pupillary  membrane.  When  the 
mesoblastic  portion  of  the  iris  is  first  forming 
it  pushes  out  beyond  the  epiblastic  portion 
and  rests  upon  the  lens  (Fig.    8).        In  this 

4 


50  THE  EMBRYOLOGY  OF    THE   EYE. 

situation  it  at  first  retains  its  original  thick- 
ness.  A  little  later  its  margin  becomes 
thinner  (Fig.  9),  and  still  later  (Figs.  11,  13) 
we  find  nothing  left  of  this  mesoblastic  out- 
growth running  to  the  vascular  sheath  of  the 
lens  but  the  endothelial  laver,  and  a  few 
spindle-cells  from  the  anterior  layer,  which 
accompany  vessels.  The  spindle-cells  extend 
only  a  short  distance  into  the  pupillary  mem- 
brane, and  in  its  central  portion  the  membrane 
consists  only  of  a  single  layer  of  capillaries, 
much  smaller  than  those  which  lie  posterior 
to  the  iris  and  carry  the  blood  from  the  vessels 
in  the  vitreous. 

At  a  late  stage  the  mesoblastic  margin  of 
the  iris  extends  out  some  distance  beyond  the 
farthest  iris  vessels,  and  capillaries  springing 
from  vessels  of  the  minor  zone  pass  from  the 
anterior  surface  of  the  iris,  some  millimetres 
from  its  margin,  and  run  to  the  pupillary 
membrane.  The  membrane  disappears  just 
before  birth,  or  just  after  birth  in  some  ani- 
mals, and  usually  there  is  no  trace  of  it  left. 


THE  EMBRYOLOGY   OF   THE  EYE.  5 1 

Any  of  the  foetal  vessels  which  are  usually 
absorbed  may,  however,  remain  permanently 
in  a  degenerated  form,  so  that  we  find  as  con- 
genital anomalies,  persistent  pupillary  mem- 
brane, in  the  form  of  plaques  on  the  capsule 
of  the  lens  or  fibres  arising  from  the  region  of 
the  minor  vascular  circle  of  the  iris  ;  persistent 
posterior  vascular  sheath  of  the  lens,  in  the 
form  of  plaques  near  the  pole  or  strlatlons 
running  merldlonally  on  the  capsule  ;  persist- 
ent hyaloid  artery,  either  its  lenticular  or 
papillary  insertion  being  preserved  ;  persistent 
vitreous  vessels  of  the  2d  system  ;  and  last, 
connective-tissue  masses  of  varying  size  In  the 
physiological  cup  of  the  papilla  (Fig.  12),  or 
running  from  this  point  out  into  the  vitreous 
or  extending  along  the  limitans  interna,  all  of 
which  anomalies  are  seen  in  the  human  eye. 

In  the  upper  portion  of  Fig.  4  we  see  how 
the  vessels  of  the  posterior  lens-sheath  run 
over  the  anterior  marorln  of  the  vesicle-wall  to 
reach  the  mesoblastic  tissue  outside.  This 
communication  remains  throughout  foetal  life, 


52  THE   EMBRYOLOGY  OF   THE  EYE. 

although  the  development  of  the  iris  changes 
the  relative  positions.  In  cases  of  coloboma 
of  the  iris  Hess^  has  in  several  cases  found  a 
fibrous  cord  extending  from  the  optic  disc  to 
the  posterior  pole  of  the  lens,  and  from  this 
point  running  meridionally  to  the  equator  of 
the  lens,  where  it  was  inserted  into  the  ciliary 
body  at  the  head  of  the  coloboma.  This  he 
regards  as  a  mesoblastic  formation,  which 
passing  in  front  of  the  margin  of  the  secondary 
vesicle  has  prevented  the  development  of  the 
iris  at  this  point. 

As  is  well  known,  in  cases  of  incomplete  or 
arrested  development  of  the  eye,  we  usually 
find  remains  of  foetal  vessels  that  have  not 
been  absorbed.  In  my  opinion,  the  cord  that 
Hess  has  described  is  a  remnant  of  the  foetal 
vessels,  which  has  remained  simply  because 
the  secondary  vesicle  has  not  developed  ante- 
-^riorly  at  some  point,  and  the  relations  seen  in 
Fig.  4  are  therefore  preserved  permanently — 
in  short,  the  persistent  cord  is  not  the  cause 

'  Grcefe's  Archizi,  xxxiv.,  3. 


THE  EMBRYOLOGY  OF   THE  EYE.  53 

of   the   coloboma  of    the  iris,   but  the  conse- 
quence. 

9.  Retina  and  pigment-epithelium, 
a.  Retina.  When  the  secondary  vesicle  is 
fully  formed  its  inner  layer  from  which  the 
retina  develops,  is  bordered  on  either  surface 
by  a  more  or  less  distinct  membrane,  which 
membranes  become  respectively  the  limitans 
externa  and  the  limitans  interna  of  the  retina 

(Fig-  4). 

The  cells  of  the  stratum  next  the  limitans 
interna  are  columnar  and  the  nucleus  lies  some 
little  distance  from  the  membrane  (Fig.  4).  A 
little  later,  these  columnar  cells  become  spindle- 
shaped  and  small  spaces  appear  between  their 
internal  prolongations.  When  the  limitans 
interna  is  stripped  off,  these  prolongations  are 
seen  to  form  a  fine  network  (Fig.  7).  A 
similar  network  is  found  on  the  margin  of  the 
wall  of  the  cerebral  vesicles.  At  the  next 
stage  we  find  nerve-fibres  extending  continu- 
ously from  the  cerebral  vesicle  through  the 
optic  nerve  to  the  retina  (Fig.  8).     The  inter- 


54  THE  EMBRYOLOGY  OE   THE  EYE. 

nal  prolongations  of  the  inner  stratum  of 
retinal  cells  just  mentioned,  may  now  be  seen 
to  be  continuous  each  with  a  nerve-fibre  (Fig. 
14).  It  is  not  possible  to  say  whether  the 
nerve-fibres  originate  peripherically  or  cen- 
trally, or  whether  they  originate  simultaneously 
at  both  extremities.  At  the  earliest  period  in 
which  I  have  found  nerve-fibres  they  are  ap- 
parently equally  developed  in  their  entire 
course. 

After  the  nerve-fibres  appear,  the  vessels 
soon  develop.  These  spring  from  the  vessels 
in  the  nerve,  and  quickly  spread  over  the  en- 
tire retina  in  the  form  of  small  capillaries  lying 
just  beneath  the  limitans  interna  (Fig.  10  at 
left). 

At  this  stage  the  differentiation  of  the  retinal 
cells  becomes  further  advanced.  The  stratum 
of  cells  next  the  limitans  externa  become 
columnar  with  long  oval  nuclei  which  stain 
deeply  (Fig.  10). 

The  mass  of  retinal  cells  have  a  round  or 
oval  nucleus,  staining  moderately  dark,  and  a 


THE  EMBRYOLOGY   OF    THE   EYE.  55 

small  amount  of  protoplasm  of  a  delicate 
spindle  form. 

The  inner  stratum  of  cells  from  which  the 
nerve-fibres  run — the  rudimentary  ganglion- 
cells  (Fig.  14,  A) — have  a  large  nucleus  at  this 
stage  staining  deeply  with  hematoxylin,  and  a 
large  cell-body  with  two  distinct  processes,  one 
continuous  with  a  nerve-fibre,  the  other  extend- 
ing back  among  the  retinal  cells. 

The  stratum  of  ganglion-cells  is  separated 
from  the  mass  of  the  retinal  cells  by  a  stratum 
of  rather  large  cells  (Fig.  14,  B),  the  nuclei  of 
which  stain  very  faintly,  and  which  a  little 
later  disappear  almost  entirely  to  give  place 
to  a  delicate  network  striated  longitudinally — 
the  inner  reticular  layer  (Fig.  15,  B).  From 
this  time  on  we  notice  long  nucleated  fibres 
staining  deeply  in  the  outer  cellular  layers 
(Figs.  14  and  15). 

The  outer  reticular  layer  is  formed  later, 
exactly  as  the  inner  layer  was  formed.  That 
is,  there  is  a  stratum  of  cells  near  the  inner 
reticular   layer  which   stain    deeply,    and  just 


56  THE  EMBRYOLOGY   OF    THE  EYE. 

external  to  this  stratum  is  a  second  in  which 
the  cells  stain  faintly  (Fig.  15,  C).  These 
faintly  staining  cells  disappear  to  give  place 
to  the  reticulum. 

Near  the  ora  serrata  the  nerve-fibre  layer  is 
very  thin,  and  only  in  this  region  can  the  sup- 
porting fibres  of  Mliller  be  studied  with  any 
degree  of  clearness.  At  a  late  stage  coarse 
fibres  may  here  be  seen  spreading  out  into  a 
cone  shape  and  inserted  into  the  limitans  in- 
terna, which  previously  has  existed  as  a  simple 
membrane.  In  this  cone-shaped  foot,  both  in 
the  foetus  and  in  the  grown  animal,  we  may 
here  and  there  distinguish  a  very  faintly  stain- 
ing nucleus. 

As  the  ganglion-cell  grows,  its  nucleus  in- 
creases greatly  in  size  and  comes  to  stain  more 
faintly.  Among  the  ganglion-cells  there  are 
from  the  beginning  small  bipolar  cells  which 
take  a  deep  stain.  These  small  cells  have  an 
oval  nucleus  with  its  long  axis  in  the  direction 
in  which  Miiller's  fibres  run.  Somewhat  simi- 
lar  cells  which    stain    less  deeply  lie  in   the 


THE  EMBRYOLOGY  OF    THE  EYE.  57 

nerve-fibre  layer,  with  the  long  axis  of  the 
nucleus  in  the  direction  of  the  nerve-fibres, 
and  are  called  glia  cells. 

At  a  very  late  period  the  rods  and  cones  are 
developed  by  the  outer  layer  of  columnar  cells 
sending  processes  through  the  limitans  ex- 
terna. In  my  specimens  from  the  foetus  and 
from  new-born  animals  this  layer  is  not  suffi- 
ciently preserved  to  justify  a  description. 

b.  The  pigment-epithelium  is  formed  from 
the  outer  epiblastic  layer  of  the  secondary  optic 
vesicle,  which  becomes  reduced  to  a  single 
stratum  of  cells  hexagonal  In  flat  section.  In 
the  chick  these  cells  are  for  a  time  all  cuboidal 
in  vertical  section  and  of  nearly  equal  size 
(Figs.  3  and  6).  In  the  pig  the  posterior  cells 
are  cuboidal,  while  the  anterior  ones  soon  be- 
come long  columnar  in  vertical  section  (Fig.  7). 
In  the  pig,  when  the  lens-sac  is  forming,  rod- 
shaped  bits  of  pigment  are  deposited  in  the 
cells  along  the  inner  margin  of  the  cells,  the 
nucleus  lying  In  the  outer  portion  (Figs.  4 
and  7).     This   deposition  of  pigment  begins 


58  THE  EMBRYOLOGY  OF   THE   EYE. 

anteriorly,  and  here  it  is  always  denser  and 
more  resistant  to  the  action  of  bleachincr 
agents  even  in  adult  life.  The  margins  of  the 
inferior  cleft  are  not  pigmented  until  a  con- 
siderable time  after  the  cleft  is  obliterated. 
The  pigment  gradually  fills  the  entire  cell,  ex- 
cepting the  nucleus  and  the  outer  margin, 
which  always  remains  free. 

In  the  chick  the  deposition  of  pigment 
occurs  somewhat  later  and  begins  in  the  outer 
portion  of  the  cell,  the  pigment  extending  to 
the  outer  margin.  Later,  when  the  anterior 
cells  become  columnar,  the  pigment  Is  most 
dense  in  the  inner  portion  of  the  cell. 

lo.  Optic  nerve. — Just  before  the  lens  is 
fully  formed,  and  while  the  eyelids  are  only 
indicated  by  two  small  folds  (Fig.  4),  w^e  find 
the  inferior  cleft  of  the  secondary  vesicle 
closed  but  still  showing  evidences  of  its  recent 
closure.  In  Fig.  16,  A  is  an  equatorial  sec- 
tion through  the  inferior  wall  of  the  vesicle, 
showing  the  inner  layer  folded  where  the  cleft 
has  closed  ;    B   is  a  section   nearer  the  optic 


THE   EMBRYOLOGY  OF   THE   EYE.  59 

nerve,  showing  the  outer  layer  folded.  It  will 
be  noticed  that  the  pigment  of  the  outer  layer 
is  wanting  in  the  region  of  the  fold.  At  a 
slightly  later  period  these  folds  are  obliterated 
and  the  pigmentation  is  uniform  in  the  outer 
layer  ;  C  Is  the  optic  stalk  near  the  vesicle, 
with  its  Inferior  cupping  ;  D  Is  the  stalk  a 
short  distance  from  the  vesicle  ;  and  E  is  the 
round  stalk  with  a  central  lumen  some  distance 
from  the  vesicle.  x\t  this  period  the  optic 
stalk  is  entirely  cellular  and  shows  no  trace  of 
nerve-fibres. 

As  w^e  have  seen,  the  cells  of  the  retina  are 
at  first  closely  packed,  but  just  before  the 
appearance  of  the  nerve-fibres  the  cells  of  the 
layer  at  the  inner  surface  of  the  retina  become 
branched,  and  the  branches  form  a  network 
and  the  cells  lie  farther  apart.  At  the  same 
time  a  similar  change  takes  place  In  the  cells 
of  the  cerebral  vesicles.  The  walls  of  the 
cerebral  vesicles  Increase  In  thickness  until  the 
cavity  Is  almost  obliterated.  The  central  cells 
are  closely  packed,  while  those  on  the  surface 


60  THE  EMBRYOLOGY  OF   THE  EYE. 

of  the  vesicle  in  places  become  branched  like 
the  retinal  cells.  From  these  branched  cells 
near  the  surface  of  the  wall  of  the  third  vesicle 
arise  the  nerve-fibres.  These  nerve-fibres  run 
anteriorly  in  a  compact  mass  on  the  surface  of 
the  vesicle,  thus  forming  the  optic  tract,  and 
then,  passing  into  the  optic  stalk,  run  without 
interruption  to  the  retina. 

In  the  optic  tract,  as  in  the  wall  of  the  cere- 
bral vesicle,  there  are  a  number  of  capillaries 
which  pass  in  from  the  surrounding  mesoblast. 
Apart  from  the  capillaries  there  are  no  connec- 
tive-tissue elements,  and  the  only  cells  in  the 
tracts  are  a  few  branched  neural-epiblastic 
cells,  mostly  lying  among  the  peripheric  fibres, 
and  identical  with  the  cells  of  the  vesicle- 
wall. 

At  the  point  of  origin  of  the  optic  nerve 
(Fig.  I  7,  A)  there  is  a  considerable  accumula- 
tion of  these  cells  among  the  nerve-fibres,  and 
passing  up  throughout  the  length  of  the  optic 
nerve  are  the  same  cells  arranged  in  short 
longitudinal    rows   which    separate   the  fibres 


THE  EMBRYOLOGY  OF    THE  EYE.  6l 

into    more   or   less    distinct    bundles,    as    the 
connective-tissue  septa  do  later. 

In  a  transverse  section  of  the  nerve  we  find 
fibres  in  every  portion,  but  grouped  into  larger 
and  more  distinct  bundles  in  the  centre. 

The  cells  among  the  fibres  throw  out  lateral 
processes  which  anastomose  so  that  a  sort  of 
loose  membrane  is  formed  grouping  the  fibres 
into  bundles  (Fig.  i8,  A).  On  the  surface  of 
the  nerve  there  is  a  layer  of  similar  cells  (Fig. 
1 8,  B)  which  have  a  large,  paler,  round  nu- 
cleus, and  well-marked  processes,  the  longer 
ones  extending  into  the  nerve,  the  shorter  ones 
joining  the  branched  and  spindle-shaped  meso- 
blastic  cells  (Fig.  i8,  C),  which  are  beginning 
to  form  a  sheath  about  the  nerve. 

At  this  stage  the  cells  of  the  mesoblast  and 
the  branched  cells  of  the  neural  epiblast  bear 
a  remarkable  resemblance  to  each  other,  and 
most  authors  have  considered  the  cells  in  the 
nerve  to  be  mesoblastic.  At  this  stage,  how- 
ever, no  mesoblastic  cells  can  be  seen  to 
extend  into  the  nerve,  and  the   cells   of   the 


62  THE  EMBRYOLOGY  OF   THE  EYE. 

nerve  are  directly  continuous  with  the  epiblas- 
tic  cells  of  the  cerebral  vesicles  and  of  the 
retina,  and  in  my  opinion  are  to  be  considered 
the  epiblastic  cells  of  the  optic  stalk. 

At  a  somewhat  later  stage  in  the  pig  I  have 
found  the  optic  nerve  much  larger,  and  con- 
sisting only  partially  of  fibres.  The  remainder 
consists  of  closely  packed  cells  continuous 
with  the  cells  of  the  cerebral  vesicle  and  of 
the  retina,  and  identical  with  the  closely 
packed  cells  found  in  these  localities  (Fig  lo). 
These  cell-masses  in  the  nerve  have  a  sharp, 
smooth  peripheric  margin  bordered  by  the 
mesoblastic  cells  of  the  nerve-sheath.  These 
can  be  nothing  but  proliferated  cells  of  the 
primitive  optic  stalk. 

A  little  later  the  optic  nerve,  which  so  far 
has  had  no  direct  blood-supply,  becomes  per- 
vaded with  capillaries  which  push  in  from  the 
sheath,  and  entering  the  nerve  run  for  the 
most  part  in  a  longitudinal  direction  (Fig.  12). 

The  nerve-fibres  have  now  begun  to  take  on 
their  medullary  sheath  and  lose  their  sharp 


THE   EMBRYOLOGY  OF    THE  EYE.  65 

At  first  the  epithelium  of  the  lid  consists  of 
a  single  layer.  The  epithelium  of  the  outer 
surface  and  of  the  margin  becomes  several 
cells  deep,  and  that  of  the  inner  surface  two 
or  three  cells  deep  (Fig.  9).  The  basal  stratum 
of  columnar  cells  stains  deeply  and  soon  be- 
comes pigmented  (Fig.  20).  Very  early  we  find 
the  external  epithelium  dipping  in  in  places 
to  form  hair  follicles  (Fig.  8).  After  the  lids 
unite  similar  indippings  at  the  margins  form 
the  follicles  for  the  cilia.  Just  behind  these, 
larger  solid  epithelial  processes  push  a  consid- 
erable distance  into  the  lids,  and  later  break 
down  in  the  centre  to  form  tubes,  which  give 
off  short  processes,  and  become  Meibomian 
glands  (Fig.   20). 

At  an  early  stage  we  find  long  fusiform  cells 
developing  into  clear  muscle-fibres  (Fig.  9), 
which  appear  first  near  the  orbital  margin  of 
the  lids  and  much  later  near  the  free  margin. 
These  muscle-fibres  are  finally  collected  into 
bundles  separated  by  loose  connective  tissue, 
and  form  the  orbicularis  muscle. 


66  THE   EMBRYOLOGY   OF    THE   EYE. 

While  the  Meibomian  glands  are  develop- 
ing, the  mesoblastic  tissue  forms  a  dense  sheath 
of  spindle-cells  about  them.  This  sheath  be- 
comes denser  and  forms  a  very  tough  fibrous 
layer  in  which  the  glands  lie  imbedded — the 
tarsus.  Between  the  tarsus  and  the  posterior 
epithelium  there  is  only  a  trace  of  loose  con- 
nective tissue,  but  beneath  the  external  epithe- 
lium there  is  a  thin  layer  of  loosely  meshed 
tissue  without  fat  cells.  The  epithelium  join- 
ing the  lid  margins  degenerates,  and  the  cilia 
pushing  through  break  it  up  and  the  lids 
separate. 

Lachrymal  apparatus.  In  the  young  foetus 
there  is  a  groove  between  the  lateral  nasal 
process  and  the  superior  maxillary  process, 
extending  from  the  eye  to  the  nostril,  and 
called  the  lachrymal  furrow  (Fig.  19,  A  and  B). 
This  furrow  soon  disappears,  and  we  see  only 
a  trace  of  its  former  existence  near  the  eye 
(Fig.  19,  C).  In  frontal  sections  at  the  stage 
shown  in  Fig.  19,  A,  B,  we  find  a  short  dis- 
tance from  the  nostril  a  thickening  of  the  epi- 


THE   EMBRYOLOGY   OF    THE   EYE.  6/ 

thelium  in  the  furrow  (Fig.  21,  A).  Farther 
toward  the  eye  we  find  an  epithelial  mass 
deep  in  the  tissue  but  still  connected  with  the 
external  epithelium  (Fig.  21,  B).  At  a  slightly 
later  stage,  the  mass  of  epithelium  is  entirely 
separated  from  the  external  epithelium  and  a 
central  channel  has  appeared  in  it,  forming 
the  lachrymal  duct. 

The  lachrymal  gland  is  formed  like  other 
secreting  glands  by  a  solid  epithelial  process 
extending  from  the  conjunctival  epithelium 
back  into  the  orbit,  and  giving  off  branches 
all  of  which  become  hollow  tubules. 


The  extensive  bibliography  may  be  found  in  the  gen- 
eral treatises  of  Manz,  Kolliker,  His,  Kessler,  and  Quain. 


EXPLANATION  OF  THE  FIGURES. 


Fig.  I. — Chick,  ist  day,  showing  at  A  A  the  primary 
optic  vesicles. 

Fig.  2. — Chick,  2d  day.     Thickening  of  external  epi- 
blast  to  form  lens. 

Fig.  3. — Chick,  3d  day.      Formation  of  lens-sac  and 
secondary  vesicle. 

Fig.  4. — Rabbit,      sagittal      section.       Lens-sac     fully 
formed. 

Fig.  5. — Rabbit  ;  early  vitreous  ;  x  200. 

Fig.  6. — Chick  ;  lens  fully  formed. 

Fig.  7. — Pig,  4  cm.  long  ;  horizontal  section. 

Fig.  8. — Pig,  4^  cm.  long  ;  horizontal  section. 

Fig.  9. — Pig,  6  cm.  long  ;  horizontal  ;  ant.  segment. 

Fig.  10. — Pig,  6  cm.  long  ;  horizontal  ;  post,  segment. 

Fig.  II. — Pig,  about  9  cm.  long  ;  horizontal  ;  ant.  seg- 
ment. 

Fig.  12. — Pig,  9  cm.  long;  horizontal;  post,  segment. 

Fig.  13. — Pig,  12.5  cm.  long  ;  horizontal. 

Fig.  14. — Retina  pig  ;  A,  ganglion-cells  ;  B,  pale  cells. 

Fig.  15. — Retina  pig  near  term  ;  A,  ganglion-cells  ;  B, 
reticular  layer  ;  C,  layer  of  pale  cells. 

68 


THE  EMBRYOLOGY  OF   THE  EYE.  69 

Fig.  16. — Pig,  3I  cm.  long  ;  A  and  B,  frontal  sections 
through  inferior  wall  just  after  closure  of  cleft  ;  C,  D,  E, 
optic  stalk. 

Fig.  17. — Pig,  d^\  cm.  long  ;  sagittal  section  optic 
nerve  ;  A,  collection  of  epiblastic  cells  among  nerve 
fibres  ;  B,  entrance  of  central  vessels. 

Fig.  18. — Optic  nerve,  pig.    A,  nerve-fibres  ;  C,  sheath. 

Fig.  19.— A  and  B,  pig,  3  cm.  long  ;  C,  pig,  4  cm. 
long. 

Fig.  20. — Pig  ;  vertical  section,  junction  of  lids  ;  M, 
Meibomian  glands  ;  S,  sweat-gland. 

Fig.  2  1. — Pig,  3  cm.  long,  frontal  section.  Lach.  duct: 
A,  near  nostril  ;  B,  near  eye. 


Fig.  I. — Chick,  first  day,  showing  at  A  A  the  primary  optic  vesicles. 


Fig.   2. — Chick,   second  day.     Thickening  of    external  epiblast  to 
form  lens. 


Fig.  4. — Rabbit,  sagittal  section.     Lens-sac  fully  formed. 


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Fig.  5. — Rabbit ;  early  vitreous  ;  highly  magnified. 


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Fig.  9. — Pig,  6  cm.  long  ;  horizontal  ant.  segment. 


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Fig.  io.— Pig,  6  cm.  long  ;  horizontal  post. 


Fig.  II.— Pig,  about  9  cm.  long;  horizontal  ant.  segment. 
A 


Fig.  12.— Pig,  g  cm.  long  ;  horizontal  post,  segment 


Fig,   14. — Retina  pig 

A,  ganglion     cells 

B,  pale  cells. 


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Fig.  15. — Retina  pig  ; 
A,  ganglion  cells  ;  B, 
reticular  layer  ;  C, 
layer  of  pale  cells. 


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Fig.  16. — Pig,  2}4  cm. 
long  ;  A  and  B. 
frontal  sections 
through  inferior  wall 
just  after  closure  of 
cleft;  C,  D,  E,  optic 
stalk. 


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Fig.  17. — Pig,  4^^  cm.  long  ;  sagittal  section 
optic  nerve  ;  A,  collection  of  epiblastic  cells 
among  nerve-fibres  ;  B,  entrance  of  central 
vessels. 


Fig.  iS. — Pig,  Optic 
nerve ;  A,  nerve  fi- 
bres ;  C,  sheath. 


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